Present commercially available resin laminating material for printed circuit boards is predominantly a bifunctional epoxy. However, the potential for a resin of this type to be utilized in a multilayer circuit board is limited because of the inherent shortcomings of bifunctional epoxy such as a low glass transition temperature (Tg) value of approximately 130 Centigrade (C) of FR-4, a poor heat resistance, a high thermal expansion coefficient and other deficiencies.
U.S. Pat. No. 4,284,877 discloses adding bismaleimide (BMI) resin to increase the Tg value of the resin formula. Bismaleimide triazine (BMT) resin also may be added to increase the Tg value of the resin formula. A drawback to this combination is that BMI resin becomes highly fragile after curing. A second drawback is that if used directly, the material is not strong enough. In the process of blending an epoxy with a BMI resin, though the Tg may be raised, after curing, the material becomes very fragile and is unfavorable for processing because of the BMI.
Next, due to the poor adhesion between BMI resin and copper foil or glass fiber, if BMI is incorporated with a polyfunctional epoxy the peel strength will decrease. An article in 5 Int. J. Adhesive and Adhesives No. 3, at 123 (July 1985) further reported that when BMI resin was added to soft rubber which modified the fragile BMI resin, adverse phenomena such as phase separation of the curing mass resulted.
To correct the drawbacks of the addition of a BMI resin, a material with a more flexible molecular structure may be introduced into a BMI resin to increase the resiliency when cured, but the resultant composition is often associated with a low Tg. See 15 Brit. Polym. J. at 2 (March 1983); British Pat. No. 1,190,718.
Finally, to achieve flame retardant standards, it is beneficial to use a flame retardant containing bromine in a conventional epoxy resin. Resin containing bromine however often exhibits a poor thermal utility. See 9 IEEE Transaction of Components, Hybrids and Man. Tech. No. 4 at 37 (December 1986); J. Macromol Sci. Chem. A. 5(7) at 1205-18 (November 1971).
For resolving the aforementioned deficiencies, the present invention discloses that through adding a modified BMI resin to a modified epoxy resin the resulting resin may maintain a high Tg value as well as the characteristics of excellent processing properties, good adhesion, flame retardancy, low bromine content and the like.
The present invention discloses a process for modifying both a BMI resin and an epoxy resin. These modified resins are then combined resulting in a varnish which can be compressed into a circuit board exhibiting the beneficial qualities enumerated above.